This invention relates, in general, to semiconductor devices, and more particularly to a low voltage, deep junction semiconductor device and method.
It is desirable to have low voltage (less than 20 volts) deep junction (20-500 microns) devices such as zener diodes and high power rectifiers for many applications. The use of a deep junction can prevent degradation of device performance often caused by a transition metal such as nickel which is applied to the wafer surface during the assembly process.
Difficultly in fabricating low voltage, deep junction devices prohibits them from being readily commercially available. It is difficult and expensive to employ a conventional blanket diffusion process to make deep junction devices since prolonged diffusion time at high temperatures is required. It is particularly difficult to fabricate a deep junction having a very high dopant concentration such as is required for a low breakdown voltage device. The dopant is generally depleted at the junction because of the distance between the diffusion source and the junction. Therefore, an extremely high initial dopant concentration is required in order to achieve a dopant concentration at the deep junction of at least 4.0.times.10.sup.16 atoms/cc which is required for a high-low junction device having a breakdown voltage of 20 volts.
Other methods of forming low voltage, deep junction devices also have several problems associated therewith. One method includes the growth of a heavily doped epitaxial layer on a heavily doped substrate of opposite conductivity type. Another includes the growth of an epitaxial layer on a heavily doped substrate followed by ion implantation of the epitaxial layer in a conductivity type opposite that of the substrate. First, epitaxial film quality is extremely poor when doped with greater than 10.sup.17 atoms/cc of an impurity which is required for various low breakdown voltage applications of high-low and symmetric junction devices. Second, a heavily doped epitaxial layer is very difficult to produce because of the hazardous nature of doping gases such as arsine, phosphine and diborane. Third, ion implantation becomes very expensive when the implant dosage is greater than 10.sup.17 atoms/cc. Accordingly, the epitaxial growth methods are typically only able to produce devices having breakdown voltages of greater than 15 volts. Furthermore, the ion implant method is generally applicable only for shallow junction (approximately 0.5 micron) devices.
In view of the above, it would be highly desirable to produce low voltage, deep junction devices that overcome the aforementioned problems.